Studies On The Metastable Phase Equilibrium For The Quinary System Li⁺(Na⁺), K⁺//Cl^-, CO₃^2-, B₄O₇^2--H₂O At 273 K

In order to comprehensive utilize the resource in the ZaBuye saline lake, Tibet, studies on the metastable equilibria at 273 K of the quinary system Li⁺(Na⁺), K⁺// Cl^-, CO₃^2-, B₄O₇^2--H₂O and four quaternary sub-systems Na⁺// Cl^-, CO₃^2-, B₄O₇^2--H₂O, Na⁺// Cl^-, CO₃^2-, SO₄^2--H₂O, K⁺// Cl^-, CO₃^2-, B₄O₇^2--H₂O and K⁺, Na⁺// Cl^-, B₄O₇^2--H₂O were done using an isothemal evaporation method. The solubilities and densities of the equilibrated solution were determined. According to the experimental data, the metastable equilibrium phase diagrams, the density-composition diagrams and the water-content diagrams were plotted.There is no double salt formed in the simple quaternary system Na⁺//Cl^-, CO₃^2-, B₄O₇^2--H₂O. The metastable phase diagram of the system consists of one invariant point, three univariant curves and three crystallization fields. The three crystallization fields correspond to the single salt Na₂CO₃·10H₂O, NaCl and Na₂B₄O₇·10H₂O. The crystallization field of Na₂B₄O₇·10H₂O is larger, but the salt NaCl has the smallest crystallization field. Na₂B₄O₇ has salting out effect to Na₂CO₃ in the solution.Burkeite 2Na2SO4·Na₂CO₃ is found in the simple quaternary system Na⁺//Cl^-, CO₃^2-, SO₄^2--H₂O. The metastable phase diagram of the system consists of two invariant points, five univariant curves and four crystallization fields. The four crystallization fields correspond to the single salt Na₂CO₃·10H₂O, NaCl Na2SO4·10H₂O and 2Na2SO4·Na₂CO₃. The crystallization field of Na2SO4·10H₂O is larger, but the salt NaCl has the smallest crystallization field. The sodium sulfate (Na2SO4) causes a weak salting-out effect on the salt sodium carbonate (Na₂CO₃).In the simple quaternary system K⁺//Cl^-, CO₃^2-, B₄O₇^2-―H₂O, no double salt was founded. The metastable phase diagram of this system consists of one invariant point, three univariant curves and three crystallization fields. The three crystallization fields correspond to single salt K2CO3·3/2H₂O, KCl and K2B4O7·4H₂O. Results show that the salt KCl has salting out effect to the salt K2B4O7 in the solution. The quaternary system Na⁺, K⁺// Cl^-, B₄O₇^2-―H₂O is of the simple type, no double salt or solid solution is formed. The metastable phase diagram of the system consists of two invariant points, five univariant curves and four crystallization fields. The four crystallization fields correspond to the single salt NaCl, KCl, K2B4O7·4H₂O and Na₂B₄O₇·10H₂O. Compared with the phase diagram of the same system at 298 K, the crystallization field of Na₂B₄O₇·10H₂O becomes bigger, while the crystallization fields of K2B4O7·4H₂O and NaCl become smaller.The results show that there is no double salt formed in the system Li⁺, K⁺// Cl^-, CO₃^2-, B₄O₇^2--H₂O, and the metastable phase diagram of the system consists of three invariant points, eleven univariant curves and six crystallization fields, The six crystallization fields correspond to single salt LiCl·H₂O, Li2CO3, LiBO2·8H₂O, KCl, K2CO3·3/2H₂O and K2B4O7·4H₂O. The crystallization fields of K2CO3·3/2H₂O and LiCl·H₂O are smaller, while the crystallization field of salt LiBO2·8H₂O is the largest. The solubility of borate is very complicated in the solution, and the salt Li[B(OH)4]·6H₂O formation mechanism also need a further research.The double salt K2CO3·Na₂CO₃·12H₂O is formed in the quinary system Na⁺, K⁺// Cl^-, CO₃^2-, B₄O₇^2--H₂O at 273 K. The metastable phase diagram of the system consists of four invariant points, thirteen univariant curves and seven crystallization fields. The seven crystallization fields correspond to the single salt NaCl, Na₂CO₃·10H₂O, Na₂B₄O₇·10H₂O, KCl, K2CO3·3/2H₂O, K2B4O7·4H₂O and double salt K2CO3·Na₂CO₃·12H₂O. The crystallization fields of Na₂B₄O₇·10H₂O and K2B4O7·4H₂O are larger, while the crystallization fields of salt K2CO3·3/2H₂O and double salt K2CO3·Na₂CO₃·12H₂O are smaller. This feature is very important to extract borate from the salt lake brine.